Modern AM broadcast stations transmit Compatible Quadrature Amplitude Modulation (C-QUAM) signals to permit reception by both standard monophonic receivers using envelope detectors and by stereo AM receivers employing stereo detectors.
As is well known in the art, a stereo signal contains a left audio component L and a right audio component R. The carrier of a C-QUAM broadcast signal is amplitude modulated by a monophonic signal, 1+L+R and is phase modulated by the stereo information in the form of (L+R) and (L-R) signals. In producing the C-QUAM broadcast signal, the carrier is first phase modulated in quadrature with the (L-R) and (L+R) audio signals. The phase modulated signal is modified by multiplying the in-phase and quadrature phase components by a factor cos .theta. where .theta. is the arc tan {(L-R)/ (1+L+R) } and is then limited to remove any amplitude variations therein. The amplitude limited output of the phase modulator is amplitude modulated by a standard 1+L+R signal in high level transmitter stages. The C-QUAM broadcast signal resulting from the foregoing operations is of the form (1+L+R)cos(2.pi.fct+.phi.) where fc is the carrier frequency, t is time and .phi. represents the phase modulation signals L+R and L-R signals with the cosine .theta. term.
A variety of stereo receivers have been implemented to accommodate C-QUAM broadcast signals. Examples of representative receivers may be found in the article entitled Quadrature System for AM Stereo by Parker, Hilbert & Sakaie published in the IEEE Transactions on Consumer Electronics, Vol. CE-23. No. 4, November 1977 at pages 456-459 and in U.S. Pat. No. 4,192,968.
The detector for a C-QUAM stereo receiver includes an envelope detector for the amplitude modulation information in the received signal, a synchronous "I" in-phase detector for the phase modulation L+R information in the received signal, a circuit for comparing the outputs of the envelope and in-phase detectors to provide a cosine correction signal, a synchronous "Q" quadrature-phase detector for the quadrature-phase modulation L-R information, a cosine correction circuit to remove the cosine information from the in-phase and quadrature-phase signals and a stereo detection circuit to detect the presence or absence of stereo signals. The standard L+R signal and L-R stereo signals obtained from the envelope and quadrature phase detectors are processed to develop left and right channel signals. In the absence of a stereo signal of acceptable strength, the stereo receiver reverts to monophonic reception and its envelope detector produces the monophonic signal L+R.
Both bipolar and CMOS type circuits have been used in C-QUAM detectors. The finite transconductance of bipolar circuits, however, introduces distortion while the multipliers used in the bipolar circuits exhibit speed and switching deficiencies and require an output level shifter and impedance buffer. CMOS type circuits eliminate the distortion and switching problems associated with the bipolar circuits but lack gain control. As a result, an additional gain control stage is needed in CMOS detectors to provide the cosine correction.